1. Field of the invention
This invention relates to a fluid treating apparatus utilizing a hollow fiber and the method of using it, particularly to an apparatus and method suited for treating blood. The apparatus is generally used for blood dialyzers, artificial lungs, plasma separators, humidifiers and the like. The explanation herein is for convenience given on the case where the apparatus is applied to an artificial lung.
2. Description of prior art
Artificial lungs have a gas exchanging function which can add oxygen to and remove carbon dioxide from the human blood, which function is one of the functions exhibited by the human lung. Bubble-type artificial lungs and membrane-type ones are currently in use.
The bubble-type artificial lung has been widely used in clinics. It however has been pointed out that with the system there tends to generate hemolysis, protein degeneration, blood clotting and minute thrombi, or activation of leukocytes or complements, and that the defoaming effect weakens upon a long-period use to thereby cause minute bubbles to invade into the blood.
The membrane-type artificial lung will contact the blood with a gas via a membrane between the two to absorb oxygen into the blood and at the same time discharge carbon dioxide into the gas. The apparatus has advantages of a smaller blood damage and a smaller priming volume compared to those with the bubble type and has hence gradually been used instead of the bubble type.
The membrane-type artificial lungs so far developed conduct gas exchange via hollow fiber membrane by using a microporous hollow fiber made of a hydrophobic polymer such as polyolefin or a homogeneous hollow fiber of a material having a gas permeability such as silicone. They consist of two types, i.e. the intracapillary flow type which comprises flowing the blood inside a hollow fiber while flowing a gas outside the hollow fiber (cf. Japanese Patent Application Laid-Open Nos. 106770/1987, 57661/1984, etc.), and the extracapillary flow type which comprises flowing a gas inside a hollow fiber and flowing the blood outside the hollow fiber (cf. Japanese Patent Application Laid-Open Nos. 57963/1984, 28806/1985, etc.).
In the intracapillary-flow type artificial lungs, when the blood is uniformly distributed inside a multiplicity of hollow fibers, the blood flow inside the hollow fiber is a complete laminar flow though there occurs no channeling (biassed flow) of the blood. The inner diameter of the hollow fiber therefore is required to be decreased for the purpose of increasing the gas exchanging ability (gas transfer rate per unit area of membrane), and hollow fibers having an inner diameter of 150 to 300.mu.m have been developed.
However, even with a small inner diameter, the gas exchanging ability cannot increase sharply insofar as the blood flows in a laminar flow. Then, the intracapillary-flow type artificial lung requires a membrane area as large as about 6 m.sup.2 for achieving a gas exchanging ability of 200 to 300 cc/min which is necessary when open heart surgery is conducted on a grown up person, and is thus large and heavy and hence difficult to handle. Furthermore, they have a large priming volume to thereby increase the burden of patients. If the inner diameter is made still smaller by compactization of the artificial lung to improve handling, there will occur frequent clotting (clogging of the inside of hollow fibers due to blood clotting). Besides, the artificial lungs of this type cannot conduct flow by gravity due to the large resistance in the blood passage, and are hence difficult for use with a blood pump of pulsation flow type. In the artificial lungs, if the gas is supplied while being not so uniformly distributed, carbon dioxide removing ability (transfer rate of carbon dioxide per unit area of membrane) will decrease; and it is difficult to distribute and supply the gas sufficiently uniformly to several thousands to several tens of thousands of hollow fibers. Special designs therefore are necessary for supplying the gas uniformly to a multiplicity of hollow fibers.
On the other hand, in the extracapillary-flow type artificial lungs, it is expected that the gas be more uniformly distributed and the blood flow have some disturbance therein. However, it has been pointed out that there tend to occur shortage of oxygenation caused by channeling of blood flow or blood clotting due to stagnation of blood flow. Commercially available extracapillary-flow type artificial lungs have a problem that there is required a large membrane area for compensating the shortage of oxygenation due to channeling of blood flow, resulting in a large priming volume. Where an artificial lung is used for a patient having a small amount of blood, blood transfusion therefore is required, which in turn leads to the danger of the patient's catching hepatis, AIDS or the like.
The present inventors have paid attention to the extracapillary-flow type artificial lungs, which suffer little pressure loss and are expected to increase their gas exchanging ability per unit area, and tried to eliminate its drawbacks, i.e. channeling and/or stagnation of blood flow by the use of a sheet of cord-fabric type comprising a multiplicity of hollow fibers connected with warps. However, it then was found that simply placing a laminate of sheets of hollow fibers connected with warps into cord-fabric type in a housing would increase, contrary to expectation, channeling of blood and decrease the gas exchanging ability per unit membrane area.